1. Field of Invention
The invention relates to an ink-jet recording apparatus.
2. Description of Related Art
An ink-jet recording apparatus includes a shear mode type ink-jet print head formed of piezoelectric material, as disclosed in U.S. Pat. Nos. 4,879,568, 4,887,100, and 5,028,936. In such an ink-jet print head, application of a voltage to the piezoelectric material causes the volumetric capacity of an ink channel to be changed. When the volumetric capacity is reduced, pressures are applied to ink in the ink channel, thereby ejecting an ink droplet from a nozzle. The ejected ink droplets impinge against a recording medium, forming text or graphics thereon.
Ink droplet volume for one dot may be easily controlled by ejecting a plurality of ink droplets for one dot. Because print density of a dot may be adjusted by changing the ink droplet volume, contrast and gradation may precisely be reproduced, improving print quality.
In the invention, drive pulse waveforms used for forming a dot with a plurality of ink droplets are changed according to whether there is an immediately preceding dot when forming a dot. Thus, the plurality of ink droplets forming a single dot may be individually ejected, without merging the ink droplets during flight.
Each of the ink droplets, flying separately, impinge against the recording medium and are slightly shifted with the relative movement of an ink-jet print head and the recording medium. Consequently, the area of a thus formed dot may be enlarged. By the above-described method, the print density may be controlled, as well as gradation may be precisely represented, by changing the number of ink droplets to be ejected.
In recent years, the need for high-speed printing and high-density printing have increased. For the high-speed printing, printing frequencies need to be raised. When the printing frequencies are raised, ink droplet ejection cycles become shortened. Therefore, after an ink droplet is ejected, vibrations remaining in the ink in an ink channel may influence the ejection of a next ink droplet. Such influences of the vibrations on the next ink droplet ejection becomes significant when the number of ink droplets to be ejected for one dot needs to be increased for the high-density printing.
The application of an invention wherein the print density is controlled as described above, may be effective to obtain a high-quality output of an image when high-speed and high-density printing is executed.
In the invention, a plurality of drive waveforms are stored in a memory. The drive waveforms are selected according to whether a first dot is printed immediately before a second dot is printed. The drive waveforms stored in the memory are different from each other. Furthermore, plurality of ink droplets are ejected to form one dot. When printing is performed based on the selected drive waveforms, each of the plurality of ink droplets, individually and sequentially, impinge against the recording medium. The ink droplets impinge against the recording medium at positions slightly shifted from each other because the print head that ejects ink droplets moves relative to the recording medium.
When a dot is formed by a plurality of ink droplets, each of which individually impinges against the recording medium, the ink droplets have a larger area than a dot formed by ink droplets merging during flight and impinging against the recording medium. Thus, areas dense with ink become large. Further, in the invention, a gap between adjacent dots is not formed, leading to the improved quality.
In the invention, regardless of whether any of the drive waveforms are used for printing, the volumes of the ink droplets ejected for one dot are the same and the shapes of the dots formed on the recording medium are the same.
Each of the drive waveforms includes a plurality of ejection pulses for the ejection of a plurality of ink droplets. Ink in an ink channel vibrates after an ink droplet is ejected. The vibrations remaining in the ink channels may have an adverse effect on the ejection of a next ink droplet. To reduce the adverse effect on the ejection of ink droplets, a stabilizing pulse to suppress the residual vibrations may be provided following the ejection pulse. The stabilizing pulse does not contribute to eject the ink, but serves to suppress the vibrations in the ink. With the stabilizing pulse, an ink droplet is stably ejected from a nozzle after the ejection of an ink droplet.
The widths of the ejection pulse and the stabilizing pulses and the intervals between the pulses may be arranged without restraint, according to the types and the viscosity of ink to be used, ambient temperatures, distances between a nozzle and a recording medium, and other conditions.